Power is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of two forms: overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings; third rail mounted at track level and contacted by a sliding "pickup shoe". Both overhead wire and third-rail systems usually use the running rails as the return conductor but some systems use a separate fourth rail for this purpose.

In comparison to the principal alternative, the diesel engine, electric railways offer substantially better energy efficiency, lower emissions and lower operating costs. Electric locomotives are also usually quieter, more powerful, and more responsive and reliable than diesels. They have no local emissions, an important advantage in tunnels and urban areas. Some electric traction systems provide regenerative braking that turns the train's kinetic energy back into electricity and returns it to the supply system to be used by other trains or the general utility grid. While diesel locomotives burn petroleum, electricity can be generated from diverse sources including renewable energy.

Disadvantages of electric traction include high capital costs that may be uneconomic on lightly trafficked routes; a relative lack of flexibility - since electric trains need electrified tracks (or overhead wires)- and a vulnerability to power interruptions. Different regions may use different supply voltages and frequencies, complicating through service and requiring greater complexity of locomotive power. The limited clearances available under overhead lines may preclude efficient double-stack container service.

Railway electrification has constantly increased in the past decades, and as of 2012, electrified tracks account for nearly one third of total tracks globally.[1]

Selection of an electrification system is based on economics of energy supply, maintenance, and capital cost compared to the revenue obtained for freight and passenger traffic. Different systems are used for urban and intercity areas; some electric locomotives can switch to different supply voltages to allow flexibility in operation.

Standardised voltages

Six of the most commonly used voltages have been selected for European and international standardisation. These are independent of the contact system used, so that, for example, 750 V DC may be used with either third rail or overhead lines.

There are many other voltage systems used for railway electrification systems around the world, and the list of railway electrification systems covers both standard voltage and non-standard voltage systems.

The permissible range of voltages allowed for the standardised voltages is as stated in standards BS EN 50163[2] and IEC 60850.[3] These take into account the number of trains drawing current and their distance from the substation.

Direct current

Railways must operate at variable speeds. Until the mid 1950s this was only practical with the brush-type DC motor, although such DC can be supplied from an AC catenary via on-board electric power conversion. Since such conversion was not well developed in the late 19th century and early 20th century, most early electrified railways used DC and many still do, particularly rapid transit (subways) and trams. Speed was controlled by connecting the traction motors in various series-parallel combinations, by varying the traction motors' fields, and by inserting and removing starting resistances to limit motor current.

Motors have very little room for electrical insulation so they generally have low voltage ratings. Because transformers (prior to the development of power electronics) cannot step down DC voltages, trains were supplied with a relatively low DC voltage that the motors can use directly. The most common DC voltages are listed in the previous section. Third (and fourth) rail systems almost always use voltages below 1 kV for safety reasons while overhead wires usually use higher voltages for efficiency. ("Low" voltage is relative; even 600 V can be instantly lethal when touched.)

Since utilities supply high-voltage AC, DC railways use converter stations to produce relatively low-voltage DC (usually 3000 volts or less). Originally they used rotary converters, a few of which are even still in operation, but most were supplanted first by mercury arc rectifiers and then by semiconductor rectifiers.

Because electrical power is equal to voltage times current, the relatively low voltages in existing DC systems imply relatively high currents. If the DC power in the contact wire is to be supplied directly to the DC traction motors, minimizing resistive losses requires thick, short supply cables/wires and closely spaced converter stations.

The distance between feeder stations on a 750 V third-rail system is about 2.5 km (1.6 mi). The distance between feeder stations at 3 kV is about 7.5 km (4.7 mi).

Because of these problems, modern high-speed rail projects have generally used high-voltage AC once the technology became available. There has, however, been interest among railroad operators in returning to DC use at higher voltages than previously used. At the same voltage, DC often has less loss than AC, and for this reason high-voltage direct current is already used on some bulk power transmission lines. DC avoids the electromagnetic radiation inherent with AC, and on a railway this also reduces interference with signalling and communications and mitigates hypothetical EMF risks. DC also avoids the power factor problems of AC. Of particular interest to railroading is that DC can supply constant power with a single ungrounded wire. Constant power with AC requires three-phase transmission with at least two ungrounded wires. Another important consideration is that mains-frequency three-phase AC must be carefully planned to avoid unbalanced phase loads. Parts of the system are supplied from different phases on the assumption that the total loads of the three phases will even out. At the phase break points between regions supplied from different phases, long insulated supply breaks are required to avoid them being shorted by rolling stock using more than one pantograph at a time. A few railroads have tried three-phase but its substantial complexity has made single-phase standard practice despite the interruption in power flow that occurs twice every cycle. An experimental 6 kV DC railway was built in the Soviet Union.

The increasing availability of high-voltage semiconductors may allow the use of higher and more efficient DC voltages that heretofore have only been practical with AC.[4]

Some DC locomotives used motor-generator sets as "stepdown transformers" to produce more convenient voltages for auxiliary loads such as lighting, fans and compressors but they are inefficient, noisy and unreliable. Solid-state converters have replaced them. State-of-the-art locomotives (diesel-electric as well as electric) have even replaced the traditional universal-type traction motor with a three-phase AC induction motor driven by a special-purpose AC inverter, a variable frequency drive.

In the United Kingdom, 1,500 V DC was used in 1954 for the Woodhead trans-Pennine route (now closed); the system used regenerative braking, allowing for transfer of energy between climbing and descending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC. It is now only used for the Tyne and Wear Metro. In India, 1,500 V DC was the first electrification system launched in 1925 in Mumbai area. Between 2012-2016, the electrification was converted to 25 kV 50 Hz AC which is the countrywide system.

Overhead systems with linear motor

Third rail

Most electrification systems use overhead wires, but third rail is an option up to 1,500 V, as is the case with Shenzhen Metro Line 3. Third rail systems exclusively use DC distribution. The use of AC is not feasible because the dimensions of a third rail are physically very large compared with the skin depth that the alternating current penetrates to 0.3 millimetres or 0.012 inches in a steel rail. This effect makes the resistance per unit length unacceptably high compared with the use of DC.[11] Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.

With top-contact third (and fourth) rail a heavy shoe attached to the underside of a woodenbeam which in turn is attached to the bogie, collects power by sliding over the top surface of the conductor rail.

Third-rail systems can be designed to use top contact, side contact, or bottom contact. Top contact is less safe, as the live rail is exposed to people treading on the rail unless an insulating hood is provided. Side- and bottom-contact third rail can easily have safety shields incorporated, carried by the rail itself. Uncovered top-contact third rails are vulnerable to disruption caused by ice, snow and fallen leaves.

Arcs like this are normal and occur when the collection shoes of a train drawing power reach the end of a section of power rail.

DC systems (especially third-rail systems) prevent the use of low-level platforms and are limited to relatively low voltages. The latter can limit the size and speed of trains, and limit the power available to passenger comforts, such as air-conditioning. The low voltage also means that long distance transmission is inefficient and thus frequent transformers are required along the length of the line. This may be a factor favouring overhead wires and high-voltage AC, even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100 mph (160 km/h) because above that speed reliable contact between the shoe and the rail cannot be maintained.

Some street trams (streetcars) used conduit third-rail current collection. The third rail was below street level. The tram picked up the current through a plough (U.S. "plow") accessed through a narrow slot in the road. In the United States, much (though not all) of the former streetcar system in Washington, D.C. (discontinued in 1962) was operated in this manner to avoid the unsightly wires and poles associated with electric traction. The same was true with Manhattan's former streetcar system. The evidence of this mode of running can still be seen on the track down the slope on the northern access to the abandoned Kingsway Tramway Subway in central London, United Kingdom, where the slot between the running rails is clearly visible, and on P and Q Streets west of Wisconsin Avenue in the Georgetown neighborhood of Washington DC, where the abandoned tracks have not been paved over. The slot can easily be confused with the similar looking slot for cable trams/cars (in some cases, the conduit slot was originally a cable slot). The disadvantage of conduit collection included much higher initial installation costs, higher maintenance costs, and problems with leaves and snow getting in the slot. For this reason, in Washington, D.C. cars on some lines converted to overhead wire on leaving the city center, a worker in a "plough pit" disconnecting the plough while another raised the trolley pole (hitherto hooked down to the roof) to the overhead wire. In New York City for the same reasons of cost and operating efficiency outside of Manhattan overhead wire was used. A similar system of changeover from conduit to overhead wire was also used on the London tramways, notably on the southern side; a typical changeover point was at Norwood, where the conduit snaked sideways from between the running rails, to provide a park for detached shoes or ploughs.

A new approach to avoiding overhead wires is taken by the "second generation" tram/streetcar system in Bordeaux, France (entry into service of the first line in December 2003; original system discontinued in 1958) with its APS (alimentation par sol – ground current feed). This involves a third rail which is flush with the surface like the tops of the running rails. The circuit is divided into segments with each segment energized in turn by sensors from the car as it passes over it, the remainder of the third rail remaining "dead". Since each energized segment is completely covered by the lengthy articulated cars, and goes dead before being "uncovered" by the passage of the vehicle, there is no danger to pedestrians. This system has also been adopted in some sections of the new tram systems in Reims, France (opened 2011) and Angers, France (also opened 2011). Proposals are in place for a number of other new services including Dubai, UAE; Barcelona, Spain; Florence, Italy; Marseille, France; Gold Coast, Australia; Washington, D.C., U.S.A.; Brasília, Brazil and Tours, France.

Fourth rail

The London Underground in England is one of the few networks that uses a four-rail system. The additional rail carries the electrical return that, on third rail and overhead networks, is provided by the running rails. On the London Underground, a top-contact third rail is beside the track, energized at 7002420000000000000♠+420 V DC, and a top-contact fourth rail is located centrally between the running rails at 2997790000000000000♠−210 V DC, which combine to provide a traction voltage of 7002630000000000000♠630 V DC. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary or a third rail.

The key advantage of the four-rail system is that neither running rail carries any current. This scheme was introduced because of the problems of return currents, intended to be carried by the earthed (grounded) running rail, flowing through the iron tunnel linings instead. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together. The problem was exacerbated because the return current also had a tendency to flow through nearby iron pipes forming the water and gas mains. Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. The four-rail system solves the problem. Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels. Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails which have to be seated on stronger metal chairs to carry the weight of trains. However, elastomeric rubber pads placed between the rails and chairs can now solve part of the problem by insulating the running rails from the current return should there be a leakage through the running rails.

On tracks that London Underground share with National Rail third-rail stock (the Bakerloo and District lines both have such sections), the centre rail is connected to the running rails, allowing both types of train to operate, at a compromise voltage of 660 V. Underground trains pass from one section to the other at speed; lineside electrical connections and resistances separate the two types of supply. These routes were originally solely electrified on the four-rail system by the LNWR before National Rail trains were rewired to their standard three-rail system to simplify rolling stock use.

Fourth-rail trains occasionally operate on the National third-rail system. To do so, the centre-rail shoes are bonded to the wheels. This bonding must be removed before operating again on fourth-rail tracks, to avoid creating a short-circuit.

Alternating current

Image of a sign for high voltage above railway electrification system

Railways and electrical utilities use AC for the same reason: to use transformers, which require AC, to produce higher voltages. The higher the voltage, the lower the current for the same power, which reduces line loss, thus allowing higher power to be delivered.

Because alternating current is used with high voltages, this method of electrification is only used on overhead wires, never on third rails. Inside the locomotive, a transformer steps the voltage down for use by the traction motors and auxiliary loads.

An early advantage of AC is that the power-wasting resistors used in DC locomotives for speed control were not needed in an AC locomotive: multiple taps on the transformer can supply a range of voltages. Separate low-voltage transformer windings supply lighting and the motors driving auxiliary machinery. More recently, the development of very high power semiconductors has caused the classic "universal" AC/DC motor to be largely replaced with the three-phase induction motor fed by a variable frequency drive, a special inverter that varies both frequency and voltage to control motor speed. These drives can run equally well on DC or AC of any frequency, and many modern electric locomotives are designed to handle different supply voltages and frequencies to simplify cross-border operation.

These low frequencies were later made unnecessary by high power locomotive rectifiers that can convert any AC frequency to DC: first the mercury-arc rectifier and then the semiconductorrectifier. Some AC railways have been converted to standard grid frequencies, but low frequencies are still widely used due to large sunken equipment costs.

Five European countries, Germany, Austria, Switzerland, Norway and Sweden, have standardized on 15 kV ​162⁄3 Hz (the 50 Hz mains frequency divided by three) single-phase AC. On 16 October 1995, Germany, Austria and Switzerland changed from ​162⁄3 Hz to 16.7 Hz which is no longer exactly one-third of the grid frequency. This solved overheating problems with the rotary converters used to generate some of this power from the grid supply.[14]

High-voltage AC overhead systems are not only for standard gauge national networks. The meter gaugeRhaetian Railway (RhB) and the neighbouring Matterhorn Gotthard Bahn (MGB) operate on 11 kV at 16.7 Hz frequency. Practice has proven that both Swiss and German 15 kV trains can operate under these lower voltages. The RhB started trials of the 11 kV system in 1913 on the Engadin line (St. Moritz-Scuol/Tarasp). The MGB constituents Furka Oberalp Bahn (FO) and Brig-Visp-Zermatt Bahn (BVZ) introduced their electric services in 1941 and 1929 respectively, adopting the already proven RhB system.

In the United States, 25 Hz, a once-common industrial power frequency, is used on Amtrak's 25 Hz traction power system at 12 kV on the Northeast Corridor between Washington, D.C. and New York City and on the Keystone Corridor between Harrisburg, Pennsylvania and Philadelphia. SEPTA's 25 Hz traction power system uses the same 12 kV voltage on the catenary in Northeast Philadelphia. This allows for the trains to operate on both the Amtrak and SEPTA power systems. Apart from having an identical catenary voltage, the power distribution systems of Amtrak and SEPTA are very different. The Amtrak power distribution system has a 138 kV transmission network that provides power to substations which then transform the voltage to 12 kV to feed the catenary system. The SEPTA power distribution system uses a 2:1 ratio autotransformer system, with the catenary fed at 12 kV and a return feeder wire fed at 24 kV. The New York, New Haven and Hartford Railroad used an 11 kV system between New York City and New Haven, Connecticut, which was converted to 12.5 kV 60 Hz in 1987.

The early systems used a low frequency (​162⁄3 Hz), and a relatively low voltage (3,000 or 3,600 volts) compared with later AC systems. The system provides regenerative braking with the power fed back to the system, so it is particularly suitable for mountain railways provided the supply grid or another locomotive on the line can accept the power.

Three-phase systems have the serious disadvantage of requiring at least two separate overhead conductors plus rail return. Locomotives operate at one, two or four constant speeds. Most modern locomotives with variable frequency drives can also do regenerative braking on both AC and DC systems and are not limited to constant speeds.

Standard frequency alternating current

Only in the 1950s after development in France (20 kV; later 25 kV) and former Soviet Railways countries (25 kV) did the standard-frequency single-phase alternating current system become widespread, despite the simplification of a distribution system which could use the existing power supply network.

The first attempts to use standard-frequency single-phase AC were made in Hungary as far back as 1923, by the Hungarian Kálmán Kandó on the line between Budapest-Nyugati and Alag, using 16 kV at 50 Hz. The locomotives carried a four-pole rotating phase converter feeding a single traction motor of the polyphase induction type at 600 to 1,100 V. The number of poles on the 2,500 hp motor could be changed using slip rings to run at one of four synchronous speeds. The tests were a success so, from 1932 until the 1960s, trains on the Budapest-Hegyeshalom line (towards Vienna) regularly used the same system. A few decades after the Second World War, the 16 kV was changed to the Russian and later French 25 kV system.

This system is quite economical but it has its drawbacks: the phases of the external power system are loaded unequally and there is significant electromagnetic interference generated as well as significant acoustic noise.

The United States commonly uses 12.5 and 25 kV at 25 Hz or 60 Hz. 25 kV, 60 Hz AC is the preferred system for new high-speed and long-distance railways, even if the railway uses a different system for existing trains.

To prevent the risk of out-of-phase supplies mixing, sections of line fed from different feeder stations must be kept strictly isolated. This is achieved by Neutral Sections (also known as Phase Breaks), usually provided at feeder stations and midway between them although, typically, only half are in use at any time, the others being provided to allow a feeder station to be shut down and power provided from adjacent feeder stations. Neutral Sections usually consist of an earthed section of wire which is separated from the live wires on either side by insulating material, typically ceramic beads, designed so that the pantograph will smoothly run from one section to the other. The earthed section prevents an arc being drawn from one live section to the other, as the voltage difference may be higher than the normal system voltage if the live sections are on different phases and the protective circuit breakers may not be able to safely interrupt the considerable current that would flow. To prevent the risk of an arc being drawn across from one section of wire to earth, when passing through the neutral section, the train must be coasting and the circuit breakers must be open. In many cases, this is done manually by the drivers. To help them, a warning board is provided just before both the neutral section and an advance warning some distance before. A further board is then provided after the neutral section to tell drivers to re-close the circuit breaker, although drivers must not do this until the rear pantograph has passed this board. In the UK, a system known as Automatic Power Control (APC) automatically opens and closes the circuit breaker, this being achieved by using sets of permanent magnets alongside the track communicating with a detector on the train. The only action needed by the driver is to shut off power and coast and therefore warning boards are still provided at and on the approach to neutral sections.

In Japanese Shinkansen lines, there are ground-operated switched sections installed instead of neutral sections. The sections detect trains running within the section and automatically switch the power supply in 0.3 s,[18] which eliminates the need to shut off power at any time.

Non-contact systems

It is possible to supply power to an electric train by inductive coupling. This allows the use of a high-voltage, insulated, conductor rail. Such a system was patented in 1894 by Nikola Tesla, US Patent 514972.[19] It requires the use of high-frequency alternating current. Tesla did not specify a frequency but George Trinkaus[20] suggests that around 1,000 Hz would be likely.

Inductive coupling is widely used in low-power applications, such as re-chargeable electric toothbrushes and more recently, mobile telephones and wearable computing devices (inductive charging). The contactless technology for rail vehicles is currently being marketed by Bombardier as PRIMOVE.[21]

Energy efficiency

Electric vs Diesel

Electric trains need not carry the weight of prime movers, transmission and fuel. This is partly offset by the weight of electrical equipment.

Regenerative braking returns power to the electrification system so that it may be used elsewhere, by other trains on the same system or returned to the general power grid. This is especially useful in mountainous areas where heavily loaded trains must descend long grades.

Central station electricity can often be generated with higher efficiency than a mobile engine/generator. While the efficiency of power plant generation and diesel locomotive generation are roughly the same in the nominal regime,[22] diesel motors decrease in efficiency in non-nominal regimes at low power [23] while if an electric power plant needs to generate less power it will shut down its least efficient generators, thereby increasing efficiency. The electric train can save energy (as compared to diesel) by regenerative braking and by not needing to consume energy by idling as diesel locomotives do when stopped or coasting. However, electric rolling stock may run cooling blowers when stopped or coasting, thus consuming energy.

Energy sources unsuitable for mobile power plants, such as nuclear power, renewable hydroelectricity, or wind power can be used. According to widely accepted global energy reserve statistics,[26] the reserves of liquid fuel are much less than gas and coal (at 42, 167 and 416 years respectively). Most countries with large rail networks do not have significant oil reserves and those that did, like the United States and Britain, have exhausted much of their reserves and have suffered declining oil output for decades. Therefore, there is also a strong economic incentive to substitute other fuels for oil. Rail electrification is often considered an important route towards consumption pattern reform.[27] However, there are no reliable, peer-reviewed studies available to assist in rational public debate on this critical issue, although there are untranslated Soviet studies from the 1980s.

AC vs DC for mainlines

Modern electrification systems take AC energy from a power grid which is delivered to a locomotive and converted to a DC voltage to be used by traction motors. These motors may either be DC motors which directly use the DC or they may be 3-phase AC motors which require further conversion of the DC to 3-phase AC (using power electronics). Thus both systems are faced with the same task: converting and transporting high-voltage AC from the power grid to low-voltage DC in the locomotive. The difference between AC and DC electrification systems lies in where the AC is converted to DC: at the substation or on the train. Energy efficiency and infrastructure costs determine which of these is used on a network, although this is often fixed due to pre-existing electrification systems. Both the transmission and conversion of electric energy involve losses: ohmic losses in wires and power electronics, magnetic field losses in transformers and smoothing reactors (inductors).[28] Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher.[29] Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for.

Comparison with diesel traction

Electric locomotives may easily be constructed with greater power output than most diesel locomotives. For passenger operation it is possible to provide enough power with diesel engines (see e.g. 'ICE TD') but, at higher speeds, this proves costly and impractical. Therefore, almost all high speed trains are electric. The high power of electric locomotives also gives them the ability to pull freight at higher speed over gradients; in mixed traffic conditions this increases capacity when the time between trains can be decreased. The higher power of electric locomotives and an electrification can also be a cheaper alternative to a new and less steep railway if trains weights are to be increased on a system.

On the other hand, electrification may not be suitable for lines with low frequency of traffic, because lower running cost of trains may be outweighed by the high cost of the electrification infrastructure. Therefore, most long-distance lines in developing or sparsely populated countries are not electrified due to relatively low frequency of trains.

Maintenance costs of the lines may be increased by electrification, but many systems claim lower costs due to reduced wear-and-tear from lighter rolling stock.[30] There are some additional maintenance costs associated with the electrical equipment around the track, such as power sub-stations and the catenary wire itself, but, if there is sufficient traffic, the reduced track and especially the lower engine maintenance and running costs exceed the costs of this maintenance significantly.

Network effects are a large factor with electrification.[citation needed] When converting lines to electric, the connections with other lines must be considered. Some electrifications have subsequently been removed because of the through traffic to non-electrified lines.[citation needed] If through traffic is to have any benefit, time consuming engine switches must occur to make such connections or expensive dual mode engines must be used. This is mostly an issue for long distance trips, but many lines come to be dominated by through traffic from long-haul freight trains (usually running coal, ore, or containers to or from ports). In theory, these trains could enjoy dramatic savings through electrification, but it can be too costly to extend electrification to isolated areas, and unless an entire network is electrified, companies often find that they need to continue use of diesel trains even if sections are electrified. The increasing demand for container traffic which is more efficient when utilizing the double-stack car also has network effect issues with existing electrifications due to insufficient clearance of overhead electrical lines for these trains, but electrification can be built or modified to have sufficient clearance, at additional cost.

Additionally, there are issues of connections between different electrical services, particularly connecting intercity lines with sections electrified for commuter traffic, but also between commuter lines built to different standards. This can cause electrification of certain connections to be very expensive simply because of the implications on the sections it is connecting. Many lines have come to be overlaid with multiple electrification standards for different trains to avoid having to replace the existing rolling stock on those lines. Obviously, this requires that the economics of a particular connection must be more compelling and this has prevented complete electrification of many lines. In a few cases, there are diesel trains running along completely electrified routes and this can be due to incompatibility of electrification standards along the route.

A problem specifically related to electrified lines are gaps in the electrification. Electric vehicles, especially locomotives, lose power when traversing gaps in the supply, such as phase change gaps in overhead systems, and gaps over points in third rail systems. These become a nuisance, if the locomotive stops with its collector on a dead gap, in which case there is no power to restart. Power gaps can be overcome by on-board batteries or motor-flywheel-generator systems.[citation needed] In 2014, progress is being made in the use of large capacitors to power electric vehicles between stations, and so avoid the need for overhead wires between those stations.[31]

More flexible primary energy source: can use coal, nuclear, hydro or wind as the primary energy source instead of oil

Disadvantages

The Royal Border Bridge in England, a protected monument. Adding electric catenary to older structures may be an expensive cost of electrification projects

Most overhead electrifications do not allow sufficient clearance for a double-stack car.

Electrification cost: electrification requires an entire new infrastructure to be built around the existing tracks at a significant cost. Costs are especially high when tunnels, bridges and other obstructions have to be altered for clearance. Another aspect that can raise the cost of electrification are the alterations or upgrades to railway signalling needed for new traffic characteristics, and to protect signalling circuitry and track circuits from interference by traction current. Electrification may require line closures while the new equipment is being installed.

Electrical grid load: adding a major new consumer of electricity can have adverse effects on the electrical grid and may necessitate an increase in the grid's power output. However, a railway can be electrified in such manner, that it has a closed and independent electrical network of its own and backup power available if the national or state electrical grid suffers from downtime.

Appearance: the overhead line structures and cabling can have a significant landscape impact compared with a non-electrified or third rail electrified line that has only occasional signalling equipment above ground level.

Fragility and vulnerability: overhead electrification systems can suffer severe disruption due to minor mechanical faults or the effects of high winds causing the pantograph of a moving train to become entangled with the catenary, ripping the wires from their supports. The damage is often not limited to the supply to one track, but extends to those for adjacent tracks as well, causing the entire route to be blocked for a considerable time. Third-rail systems can suffer disruption in cold weather due to ice forming on the conductor rail.[33]

Theft: the high scrap value of copper and the unguarded, remote installations make overhead cables an attractive target for scrap metal thieves.[34] Attempts at theft of live 25 kV cables may end in the thief's death from electrocution.[35] In the UK, cable theft is claimed to be one of the biggest sources of delay and disruption to train services — though this normally relates to signalling cable, which is equally problematic for diesel lines.[36]

People may climb onto standing train cars, and some are seriously hurt or killed when they come too close to the overhead contact line.[37][38]

Birds may perch on parts with different charges, and animals may also touch the electrification system. Animals fallen to the ground are fetched by foxes or other predators.[39]

In most of the world's railway networks, the height clearance of overhead electrical lines is not sufficient for a double-stack container car.

World electrification

In 2006, 240,000 km (150,000 mi) (25% by length) of the world rail network was electrified and 50% of all rail transport was carried by electric traction.

Sparks effect

Newly electrified lines often show a "sparks effect", whereby electrification in passenger rail systems leads to significant jumps in patronage / revenue.[42] The reasons may include electric trains being seen as more modern and attractive to ride,[43][44] faster and smoother service,[42] and the fact that electrification often goes hand in hand with a general infrastructure and rolling stock overhaul / replacement, which leads to better service quality (in a way that theoretically could also be achieved by doing similar upgrades yet without electrification). Whatever the causes of the sparks effect, it is well established for numerous routes that have electrified over decades.[42][43]

Diesel island

A "diesel island" is a relatively short section of non-electrified railway which is surrounded by or connected to electrified lines. They are called diesel "islands" because generally they can only be used by diesel trains. Such sections pose a disadvantage in terms of operation, because electric trains from the surrounding lines cannot run on the isolated diesel section.

^It turns out that the efficiency of electricity generation by a modern diesel locomotive is roughly the same as the typical U.S. fossil-fuel power plant. The heat rate of central power plants in 2012 was about 9.5k BTU/kwh per the Monthly Energy Review of the U.S. Energy Information Administration which corresponds to an efficiency of 36%. Diesel motors for locomotives have an efficiency of about 40% (see Brake specific fuel consumption, Дробинский p. 65 and Иванова p.20.). But there are reductions needed in both efficiencies needed to make a comparison. First, one must degrade the efficiency of central power plants by the transmission losses to get the electricity to the locomotive. Another correction is due to the fact that efficiency for the Russian diesel is based on the lower heat of combustion of fuel while power plants in the U.S. use the higher heat of combustion (see Heat of combustion. Still another correction is that the diesel's reported efficiency neglects the fan energy used for engine cooling radiators. See Дробинский p. 65 and Иванова p.20 (who estimates the on-board electricity generator as 96.5% efficient). The result of all the above is that modern diesel engines and central power plants are both about 33% efficient at generating electricity (in the nominal regime).

^Per Railway electrification in the Soviet Union#Energy-Efficiency it was claimed that after the mid 1970s electrics used about 25% less fuel per ton-km than diesels. However, part of this savings may be due to less stopping of electrics to let opposing trains pass since diesels operated predominately on single-track lines, often with moderately heavy traffic.